Index: Lib/tarfile.py
===================================================================
--- Lib/tarfile.py	(revision 46822)
+++ Lib/tarfile.py	(working copy)
@@ -1010,11 +1010,16 @@
 
         if mode in ("r", "r:*"):
             # Find out which *open() is appropriate for opening the file.
+            if (fileobj):
+                orig_pos = fileobj.tell() # in case a failed opener moves it
+
             for comptype in cls.OPEN_METH:
                 func = getattr(cls, cls.OPEN_METH[comptype])
                 try:
                     return func(name, "r", fileobj)
                 except (ReadError, CompressionError):
+                    if (fileobj): # test failed, restor the file position
+                        fileobj.seek(orig_pos)
                     continue
             raise ReadError("file could not be opened successfully")
 
Index: Lib/test/test_tarfile.py
===================================================================
--- Lib/test/test_tarfile.py	(revision 46822)
+++ Lib/test/test_tarfile.py	(working copy)
@@ -3,6 +3,7 @@
 import shutil
 import tempfile
 import StringIO
+import random
 
 import unittest
 import tarfile
@@ -524,7 +525,36 @@
         self.assertEqual(tarinfo.type, tarfile.REGTYPE,
                 "dereferencing hardlink failed")
 
+class ShuffleDict(dict):
+    def __iter__(self):
+        keys = self.keys()
+        random.shuffle(keys)
+        return iter(keys)
 
+class OpenOrderTest(BaseTest):
+    """ make sure that we can try all the openers in OPEN_METH and
+        one doesn't step on the toes of another.
+    """
+    shuffle_tries = 3 # min number that triggers the error 90%+
+    def test_opener(self):
+        # test as many formats as possible
+        testable = [tarname()]
+        if gzip:
+            testable.append(tarname('gz'))
+        if bz2:
+            testable.append(tarname('bz'))
+
+        orig_OPEN_METH = tarfile.TarFile.OPEN_METH
+        tarfile.TarFile.OPEN_METH = ShuffleDict(orig_OPEN_METH)
+        try:
+            for (dummy) in range(self.shuffle_tries):
+                for (testfile) in testable:
+                    print "FOO", dummy, testfile
+                    fob = open(testfile, 'rb')
+                    tarfile.open('', 'r', fileobj=fob)
+        finally:
+            tarfile.TarFile.OPEN_METH = orig_OPEN_METH
+
 # Gzip TestCases
 class ReadTestGzip(ReadTest):
     comp = "gz"
@@ -606,6 +636,7 @@
         WriteStreamTest,
         WriteGNULongTest,
         ReadGNULongTest,
+        OpenOrderTest,
     ]
 
     if hasattr(os, "link"):